Thursday, November 26, 2009

CHEMICAL SOLUTION


Summary and Introduction

Nearly every chemical reaction takes place in homogeneous mixtures called solutions. Therefore, we must understand the properties of solutions before we can even begin to understand those reactions. Perhaps the most salient characteristic of a solution is its concentration--how much solute is dissolved in what amount of solvent. Several different units of concentration like mass percent, mole fraction, molarity, normality, and molality have been developed for use in different situations. Solution Composition explains the definitions and uses of those units and why it is necessary to have so many different units of concentration.

After we have discussed the units of concentration, we will explore the questions of why solutions form at all and what factors affect the solubility of solutes in different solvents. As we shall see, like dissolves like. Non-polar solvents dissolve non-polar solutes better than polar solvents and polar solvents dissolve polar solutes better than non-polar solvents. Raising the temperature of a solution will increase the solubility of most solid solutes. Likewise, according to Henry's law, increasing the pressure above a solution will increase the solubility of most gaseous solutes.

There are several other properties of solutions, besides concentration and solubility, called colligative properties, that depend only on the number of solute particles, not on their identities. The colligative properties that we will study are vapor pressure lowering, freezing point depression, boiling point elevation, and osmotic pressure. Each of those properties is discussed in detail in Colligative Properties.

Terms

Colligative Property - A property that depends only on the number of solute molecules in a solution and not the identity of the solute.
Concentration - The amount of solute dissolved in a given amount of solvent.
Henry's Law - The solubility of a gas in a solvent is proportional to the partial pressure of the gas above the solvent.
Mass Percent - The number of grams of solute divided by the number of grams of solution multiplied by 100%.
Molar Equivalent - The number of molar equivalents depends on the reaction of interest. For an acid-base reaction the number of equivalents is the number of moles of acid or base needed to fully neutralize the solute of interest. Sulfuric acid is a diprotic acid, therefore, 1 mole of sulfuric acid is 2 molar equivalents. For a redox reaction, the number of molar equivalents is the number of moles of electrons that one mole of the solute can either donate or accept in the particular redox reaction of interest.
Molality - The number of moles of solute divided by the number of kilograms of solvent.
Molarity - The number of moles of solute divided by the number of liters of solution.
Mole Fraction - The number of moles of solute divided by the total number of moles in solution.
Non-polar - A molecule with a low net dipole.
Normality - The number of molar equivalents of solute divided by the number of liters of solvent.
Polar - A molecule with a large net dipole.
Solubility - The amount of a particular solute that can dissolve in a given amount of a particular solvent. Solubilities are generally listed in g / L.
Solute - A minor component of a solution.
Solution - A homogeneous mixture.
Solvent - The major component of a solution.


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